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Introduction

During Late Miocene to Early Pliocene time (c. 7.2–5 Ma) an extensive fluvial and coastal sandplain developed across the Murray Basin in response to regional marine regression due to falling sea levels combined with gentle tectonic uplift. Sand ridges preserved across the sandplain record coastal shorelines formed at periods of highstand and stillstand during overall retreat of the sea towards the southwest (Fig. 1). The paleocoastal beach dunes and associated shallow offshore sands have been a focus of exploration for heavy mineral (HM) deposits since discovery, in 1983, of large resources of fine-grained heavy minerals at WIM 150, near Horsham, Victoria, followed in the mid-1990s by discovery of commercial grades of coarse-grained heavy mineral sands at Wemen, Woornack and Kulwin, southeast of Mildura. In 1989, heavy mineral discoveries were made in the South Australian portion of the basin at Mindarie and Perponda. The Mindarie deposits were subsequently developed by Australian Zircon NL (2007–2009) and Murray Zircon Pty Ltd (2012–2015). Combined resources (measured, indicated and inferred), across 11 deposits reported by Murray Zircon to January 2016, totalled 244 Mt at 3.1% (total HM), with valuable heavy mineral composition averaging 17.4% zircon, 5.0% rutile, 7.4% leucoxene and 44.4% ilmenite (Murray Zircon Pty Ltd 2016). Mining operations at Mindarie were suspended in March 2015 as a consequence of lower prices for zircon and rutile, and depletion of heavy mineral resources within economic pumping distance of the primary concentration plant, located 1.5 km north of Mindarie township. Investigation of the provenance of zircon in heavy mineral deposits in the Mindarie area was initiated to identify the relative contribution of heavy minerals from various possible source regions. The results provide data that can be used to evaluate reconstructions of the paleocoastal environment and also to assess the influence of variation in source region as a factor affecting the grade and quality of the economic heavy minerals.

Samples, analyses and results

Ten samples from paleostrandlines of Loxton Sand between Loxton and Karoonda (Fig. 2) were selected for analysis (Table 1). Samples were taken mostly from anomalous heavy mineral intersections in company reverse circulation exploration drillholes. Zircons were separated using heavy liquid and magnetic techniques. Quantitative analyses of U and Pb isotopes within individual zircon grains were determined by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) at Adelaide Microscopy, University of Adelaide. Up to 100 individual grains were analysed from each sample. Methodology and full results of analyses and age determinations are provided in Keeling et al. (2016). Results of previous work on dating populations of Murray Basin zircons (Sircombe 1999) were combined with zircon age data from likely source regions within the wider Murray Basin catchment (e.g. Veevers 1984; Ireland et al. 1998; Champion et al. 2009) to establish age groupings that were most usefully linked to possible source regions (Table 2). The relative proportion of zircons reporting to a particular age range and the mix of zircon populations for individual samples were used to evaluate likely dispersion pathways and mixing of source regions. The change in relative proportions of zircon populations between strandlines was also examined to assess variation in source inputs during progradation of the sandplain.

Zircon source and significance

All samples contained a high proportion of zircons younger than c. 480 Ma; minimum contribution ranged from 31 to 54%. In this region of the Murray Basin, zircons younger than 480 Ma were unlikely to have been contributed from reworking offshore sediments or from the western headland of Adelaide Fold Belt rocks (Fig. 3). Rather, they were delivered to the coastline by fluvial transport and subsequently reworked by wave action and longshore drift under westerly to southwesterly weather systems to concentrate as coastal residual deposits. Included in this Phanerozoic grouping was a significant population of zircons in the age range 440–350 Ma. The source of these zircons is attributed to the largely igneous second phase of the Lachlan Orogen, which produced granitoids over the period 440–360 Ma (Sircombe 1999). Zircons of this age dominate in the WIM 150 heavy mineral deposit and were sourced from southern central Victoria, accompanied by input of southern New England Orogen zircons with ages 480–440 Ma. The proportion of zircons from these two age groupings in the South Australian samples is higher than that reported for northerly and westerly Murray Basin samples from Spring Hill, Hispanola and Robinvale (Sircombe 1999). This provides the clearest evidence of fluvial transport from the southeast toward the western Murray Basin margin. Such a fluvial system was most likely the Miocene beginnings of an ancestral Murray River that developed as the sea retreated.

The population of Jurassic to Early Cretaceous (175–100 Ma) zircons is correlated with extensive volcanic arc activity of the Whitsunday Volcanic Province, mostly during the period 132–95 Ma along the eastern Queensland coast (Sircombe 1999; Bryant et al. 2012). Zircons in weathered detritus from volcanic deposits were extensively dispersed during the Cretaceous by westerly and southwesterly flowing rivers that deposited sediment in the Surat and Eromanga basins (Veevers 1984; Bryant et al. 2012). The sediments were later reworked by early Cenozoic rivers, including southerly flows into the northern Murray Basin. During the Miocene, tributaries of the ancestral Darling River transported zircon directly from Queensland volcanics and also recycled zircon from earlier basin sediments. Early Cretaceous age zircons were not recorded in the WIM 150 Miocene sands, in the southern Murray Basin, but were present in northerly samples reported by Sircombe (1999). Using data from Sircombe (1999), the relative significance of northern fluvial inputs was estimated for each of the South Australian samples. The overall results suggest that an ancestral Murray River mostly dominated over the ancestral Darling River as a source of fluvial zircons in the Loxton–Karoonda strandlines (e.g. Mindarie C dune, sample 217308). The exception was Wunkar 7 (sample 217303), which recorded the highest percentage of 175–100 Ma zircons (10%), indicative of dominant northern fluvial input (Fig. 4).

Zircon contributions from Neoproterozoic Adelaidean sedimentary rocks of the southern Adelaide Fold Belt were most probably fluvial inputs, principally from the northwest. Based on data from Ireland et al. (1998) the dominantly Umberatana Group sediments would contribute a zircon population centred on c. 1140 Ma coupled with zircons from age groupings 1900–1550 Ma and 2700–2100 Ma. Some contribution from this combination of ages was evident in most of the strandline sample data, but at a lower proportion than might be expected given the proximity of these rocks to the western basin margin. The contribution of 1900–1550 Ma zircons ranged from 2 to 10% and 2700–2100 Ma zircons from 1 to 7%; the 1140 Ma population is present also in Kanmantoo Group and, in isolation, is not diagnostic of source. Overall, Neoproterozoic rocks contributed probably <30% of the strandline zircons.

Kanmantoo Group metasandstones are characterised by a main zircon population between 700–500 Ma, with a subordinate population between 1200–900 Ma and scattered older zircon ages of 3500–2000 Ma (Ireland et al. 1998). Additional contribution of zircon grains from coastal headlands of Kanmantoo Group and offshore Delamerian granite sources was indicated by an increase in the zircon population between 700–500 Ma coupled with an increase for 1200–900 Ma zircons, and a contribution from Delamerian granites at 530–480 Ma. Two samples possibly show this pattern, Mindarie A (217306) and Balmoral South strandline (217310; Fig. 4); both samples recorded the lowest percentage of zircons younger than 480 Ma, at 31% and 32% respectively.

Conclusion

Zircon age populations indicate largely fluvial transport to the western Murray Basin Late Miocene – Early Pliocene coastline by a combination of paleodrainage networks involving the ancestral Darling and Murray river systems. Sediment from the southeast mostly dominated over that from the north. It is likely, therefore, that an ancestral Murray River was established as a source of sediment supply by the time coastal progradation had extended to the south of Loxton. Significant but subordinate fluvial input is indicated for drainage from Adelaide Fold Belt rocks forming the western basin margin. In particular, samples from strandlines Mindarie A and Balmoral South contain a higher proportion of zircons from age populations consistent with additional inputs from Kanmantoo Group metasandstones eroded from coastal headlands or offshore islands. This may reflect periods of low river flows or reactivated fault activity on the western basin margin. Overall, the Loxton–Karoonda area of the Miocene Murray Basin received zircons from a wide range of source regions, with variable inputs as the coastline prograded. The mix of zircon sources and fluctuation in source inputs with time is expected to be reflected also in variation in the characteristics and quality of zircon product and associated heavy minerals.